IF NOT EXISTS
(
	SELECT *
	FROM dbo.[sysobjects]
	WHERE [id]=OBJECT_ID(N'dbo.[ExampleTable.ColumnName.UniqueKey]') AND OBJECTPROPERTY(id, N'IsUniqueCnst')=1
)
	BEGIN
	ALTER TABLE dbo.[ExampleTable] ADD CONSTRAINT [ExampleTable.ColumnName.UniqueKey] UNIQUE NONCLUSTERED ([ColumnName])
	END
GO